Navien Internal Circulation Pump Pipe Noise

I have an NPE-240A that was installed two weeks ago. I have it set for internal intelligent recirculation mode and I've experienced a significant amount of noise being transmitted through my cold water inlet pipe (3/4" copper), but only during recirculation, not normal operation. I can hear this noise two floors away.

1. I put the unit into pump test mode (Diagnostics for 5 seconds, select pump, press i) and observed the noise occurs. To me this confirms it's the recirc function, not the rest of the heating functions. Also, observationally it's whisper quiet when running.
   
2. I confirmed there was no air trapped in the pump by opening the red cap (I heard it when it first ran so I'm pretty sure I know what air in the pump sounds like).
   
3. I closed the cold water inlet valve while the pump was running and I observed that the noise went away. The pump is quite quiet when running.
4. I loosened the pipe hangers to confirm it wasn't radiating through the structure.
5. I confirmed both filters are clean and the internal check valve moves freely.
6. I installed an inline check valve on the cold water inlet hoping to replicate the closed water inlet valve, but the noise continues.
7. I called Level 2 support and they were quite helpful but ultimately asked me to turn my water pressure down 20 psi and try it for a day. I confirmed that turning the pressure down does not improve the noise.

Any ideas how to move forward with this? I'm 90% certain there's no problem with the unit, and if so what can I do to minimize noise transmission in my pipes?

 

I would want to know more about the recirculation piping.
I've got dozens of NPE -A installed with dedicated recir. and cannot imagine why you are hearing anything on the cold water piping.
The recirculation piping is only related to the hot and recirc taps, with the exception of the internal check valve.
Adding an external check valve further isolated the cold flow.
So, I'm definitely as puzzled as you are.

 

To answer your question...it's entirely internal! There is no dedicated recirc line nor am I using a NaviCirc. The INT/EXT valve is set to INT and dip switches 1 and 2 are ON (Internal, Intelligent). Possibly I've got a bum pump, but the Level 2 rep asked for the GPM in test mode (4.4 GPM) and declared it working perfectly.

 

Update: I installed sound isolating pipe hangers and no improvement. Support is sending me a new pump to try.